Polyacetylene composites

In this paper, we explore the morphological features of polyacetylene/polybutadiene (PA/PB) blends in detail via electron microscopic techniques. We have also extended our original study of stress induced property enhancement in blends with polyacetylene compositions in the 40-60% range (Blend paper 1, ref. 5). 

When acetylene gas is polymerized in a solid solution of the Shirakawa catalyst and polybutadiene, a heterogenous blend consisting of a amorphous polybutadiene phase and a crystalline polyacetylene phase is formed. (5) The mechanical and electrical properties of this composite are critically dependent on the composition of the blend components and on their relative arrangement. In our initial Blend paper, (5) for example, we showed that the mechanical properties of PA/PB blends are a function of the blend composition, with low polyacetylene compositions ex-... 

As expected, the electrical conductivity of the doped blend is also a function of the polyacetylene composition of the material. (5) Furthermore, stretch induced elongation of the blends leads to a dramatic increase in conductivity subsequent to doping, further confirming that the electrical properties are also very sensitive to the arrangement of the respective phases. 

For all compositions, the polyacetylene domains are crystalline as is revealed by X-ray diffraction. This is further confirmed by electron diffraction from thin microtomed sections of blends with polyacetylene compositions of 40% or higher. Micro-toming of samples with less than 40% polyacetylene without sample cooling is difficult due to the rubbery nature of the composite. The polycrystalline nature of the polyacetylene domains is established from the observation that even selected area aperture of a few hundred angstroms produces Debye ring patterns. 

J. Steinmetz, H.J. Lee, S. Kwon, D.S. Lee, C. Goze-Bac, E. Aboul-Hamad, H. Kim, Y.W. Park, J.Y. Choi, Routes to the synthesis of carbon nanotube-polyacetylene composites by Ziegler-Natta polymerization of acetylene inside carbon nanotubes, Curr. Appl. Phys., 7, 39 1 (2007). 

There are also other approaches to polymer stabilization (Aldiss 1989). For example, it was found that the formation of composites of two conducting polymers, one of which is air stable, improves the stability of polymer materials. Experiments carried out with pyrrole/polyacetylene and polyaniline/ polyacetylene composites have shown that the composites appeared to be more stable than doped polyacetylene and possessed mechanical properties similar to polyacetylene. Stabilization can also be achieved chemically by copolymerization. In particular, it was found that copolymerization of acetylene with other monomers such as styrene, isoprene, ethylene, or butadiene was accompanied by the increase of improvement of polymer stability (Aldiss 1989). Crispin et al. (2003) established that... 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Itoh, A., Sasaki, K., Mizukami, H., Ohashi, H., Sakurai, T. and Hiraoka, N. 1997. Geographical variation in the furanocoumarin and polyacetylenic compound compositions of wild Glehnia littoralis plants. Nat. Med. 51 50-55. 

Most carbon fibers use PAN as their precursor however, other polymer precursors, such as rayon [8], pitch (a by-product of petroleum or coal-coking industries), phenolic resins, and polyacetylenes [6,7], are available. Each company usually uses different precursor compositions for its products and thus it is difficult to know the exact composition used in most commercially available carbon fiber products. 

Hydroxy-terminated polyester (HTPS) is made from diethylene glycol and adipic acid, and hydroxy-terminated polyether (HTPE) is made from propylene glycol. Hydroxy-terminated polyacetylene (HTPA) is synthesized from butynediol and paraformaldehyde and is characterized by acetylenic triple bonds. The terminal OH groups of these polymers are cured with isophorone diisocyanate. Table 4.3 shows the chemical properties of typical polymers and prepolymers used in composite propellants and explosives.E4 All of these polymers are inert, but, with the exception of HTPB, contain relatively high oxygen contents in their molecular structures. 

Table 12 shows the composition of the S-SBR compound used for these investigations. The sulfur used was either unmodified or one of the four samples of polyacetylene-coated sulfur described in Table 3. 

Figure 29.21 The upper portion shows the EPR lines recorded prior to exposing polyacetylene film to gaseous AsFs and the transformation of a symmetric EPR line shape to a dysonian line for conductive material. The ratio of the low-field to high-field amplitudes is shown in the inset as a function of the composition of the doped film. The lower portion of the curve shows a single line fit to the theoretical dysonian line shape (circles). 

The electrochemical polymerization of pyrrole or thiophene readily lends itself to formation of composites. Polypyrrole-acetylene laminates have been made by using polyacetylene as an electrode 295). The polypyrrole forms as a 5 pm skin on the polyacetylene. If the polyacetylene is first doped, the polypyrrole completely permeates the film. In both cases the conductivity of the composite reached 30-40 S cm-1 and was much less sensitive than that of pure polyacetylene to exposure to moist air or water, so that the polypyrrole protects the polyacetylene even in the case where it permeates the film. In this latter case, treatment with ammonia caused the conductivity to drop by 30 x whereas for the sandwich films the conductivity dropped by 4600 x through the film but only 17 x in the surface layers. 

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

Mass spectrometry has proved extremely useful in determining the composition of various polyferrocene products such as those formed by trimerization of ferrocenylacetylenes (180), reactions of ferrocenyl-acetylenes with metal carbonyls (175), oxidative coupling of ferrocenyl polyacetylenes (179), and lithiation of ferrocene (186). The 1,12-dimethyl-[l,l]ferrocenophane (XC) shows a very strong molecular ion and fragments [M—Me]+ [M—2 Me]+ and the doubly charged species M2+, [M —Me]2+, and [M —2 Me]2+ (197). 

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